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zephyr: Add support for embedded AES key
The commit provides Kconfig options that allow to configure MCUboot to use embedded AES key. Primary option is CONFIG_BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY that allows to select usage of embedded key in the code. After it follow sets of Kconfigs: - CONFIG_BOOT_ENCRYPT_IMAGE_GENERATE_BASIC_KEY_PROVIDER - CONFIG_BOOT_ENCRYPT_IMAGE_USE_CUSTOM_KEY_PROVIDER The above set allows to select source of the key. The first option will choose to generate default key provider, with a single embedded key, where the key is provided as a string assigned to CONFIG_BOOOT_ENCRYPT_IMAGE_EMBEDDED_RAW_KEY. The second option selects user provided code as source of key(s). Signed-off-by: Dominik Ermel <[email protected]>
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‎boot/zephyr/CMakeLists.txt‎

Lines changed: 63 additions & 46 deletions
Original file line numberDiff line numberDiff line change
@@ -393,57 +393,59 @@ if(NOT CONFIG_BOOT_SIGNATURE_KEY_FILE STREQUAL "")
393393
endif()
394394

395395
if(CONFIG_BOOT_ENCRYPTION_KEY_FILE AND NOT CONFIG_BOOT_ENCRYPTION_KEY_FILE STREQUAL "")
396-
# CONF_FILE points to the KConfig configuration files of the bootloader.
397-
unset(CONF_DIR)
398-
foreach(filepath ${CONF_FILE})
399-
file(READ ${filepath} temp_text)
400-
string(FIND "${temp_text}" ${CONFIG_BOOT_ENCRYPTION_KEY_FILE} match)
401-
if(${match} GREATER_EQUAL 0)
402-
if(NOT DEFINED CONF_DIR)
403-
get_filename_component(CONF_DIR ${filepath} DIRECTORY)
404-
else()
405-
message(FATAL_ERROR "Encryption key file defined in multiple conf files")
396+
if(NOT CONFIG_BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY)
397+
# CONF_FILE points to the KConfig configuration files of the bootloader.
398+
unset(CONF_DIR)
399+
foreach(filepath ${CONF_FILE})
400+
file(READ ${filepath} temp_text)
401+
string(FIND "${temp_text}" ${CONFIG_BOOT_ENCRYPTION_KEY_FILE} match)
402+
if(${match} GREATER_EQUAL 0)
403+
if(NOT DEFINED CONF_DIR)
404+
get_filename_component(CONF_DIR ${filepath} DIRECTORY)
405+
else()
406+
message(FATAL_ERROR "Encryption key file defined in multiple conf files")
407+
endif()
406408
endif()
407-
endif()
408-
endforeach()
409+
endforeach()
409410

410-
if(IS_ABSOLUTE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
411-
set(KEY_FILE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
412-
elseif((DEFINED CONF_DIR) AND
413-
(EXISTS ${CONF_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE}))
414-
set(KEY_FILE ${CONF_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
415-
else()
416-
set(KEY_FILE ${MCUBOOT_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
417-
endif()
418-
message("MCUBoot bootloader encryption key file: ${KEY_FILE}")
411+
if(IS_ABSOLUTE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
412+
set(KEY_FILE ${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
413+
elseif((DEFINED CONF_DIR) AND
414+
(EXISTS ${CONF_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE}))
415+
set(KEY_FILE ${CONF_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
416+
else()
417+
set(KEY_FILE ${MCUBOOT_DIR}/${CONFIG_BOOT_ENCRYPTION_KEY_FILE})
418+
endif()
419+
message("MCUBoot bootloader encryption key file: ${KEY_FILE}")
420+
421+
# Emit a warning if using one of the default MCUboot key files
422+
set(mcuboot_default_encryption_files
423+
${MCUBOOT_DIR}/enc-ec256-priv.pem
424+
${MCUBOOT_DIR}/enc-ec256-pub.pem
425+
${MCUBOOT_DIR}/enc-rsa2048-priv.pem
426+
${MCUBOOT_DIR}/enc-rsa2048-pub.pem
427+
${MCUBOOT_DIR}/enc-x25519-priv.pem
428+
${MCUBOOT_DIR}/enc-x25519-pub.pem
429+
)
419430

420-
# Emit a warning if using one of the default MCUboot key files
421-
set(mcuboot_default_encryption_files
422-
${MCUBOOT_DIR}/enc-ec256-priv.pem
423-
${MCUBOOT_DIR}/enc-ec256-pub.pem
424-
${MCUBOOT_DIR}/enc-rsa2048-priv.pem
425-
${MCUBOOT_DIR}/enc-rsa2048-pub.pem
426-
${MCUBOOT_DIR}/enc-x25519-priv.pem
427-
${MCUBOOT_DIR}/enc-x25519-pub.pem
428-
)
431+
if(${KEY_FILE} IN_LIST mcuboot_default_encryption_files)
432+
message(WARNING "WARNING: Using default MCUboot encryption key file, this file is for debug use only and is not secure!")
433+
endif()
429434

430-
if(${KEY_FILE} IN_LIST mcuboot_default_encryption_files)
431-
message(WARNING "WARNING: Using default MCUboot encryption key file, this file is for debug use only and is not secure!")
435+
set(GENERATED_ENCKEY ${ZEPHYR_BINARY_DIR}/autogen-enckey.c)
436+
add_custom_command(
437+
OUTPUT ${GENERATED_ENCKEY}
438+
COMMAND
439+
${PYTHON_EXECUTABLE}
440+
${MCUBOOT_DIR}/scripts/imgtool.py
441+
getpriv
442+
-k
443+
${KEY_FILE}
444+
> ${GENERATED_ENCKEY}
445+
DEPENDS ${KEY_FILE}
446+
)
447+
zephyr_library_sources(${GENERATED_ENCKEY})
432448
endif()
433-
434-
set(GENERATED_ENCKEY ${ZEPHYR_BINARY_DIR}/autogen-enckey.c)
435-
add_custom_command(
436-
OUTPUT ${GENERATED_ENCKEY}
437-
COMMAND
438-
${PYTHON_EXECUTABLE}
439-
${MCUBOOT_DIR}/scripts/imgtool.py
440-
getpriv
441-
-k
442-
${KEY_FILE}
443-
> ${GENERATED_ENCKEY}
444-
DEPENDS ${KEY_FILE}
445-
)
446-
zephyr_library_sources(${GENERATED_ENCKEY})
447449
endif()
448450

449451
if(CONFIG_MCUBOOT_CLEANUP_ARM_CORE)
@@ -731,3 +733,18 @@ if(SYSBUILD)
731733
set(mcuboot_image_footer_size ${required_size} CACHE INTERNAL "Estimated MCUboot image trailer size" FORCE)
732734
set(mcuboot_image_upgrade_footer_size ${required_upgrade_size} CACHE INTERNAL "Estimated MCUboot update image trailer size" FORCE)
733735
endif()
736+
737+
if(${CONFIG_BOOT_ENCRYPT_IMAGE_GENERATE_BASIC_KEY_PROVIDER})
738+
# Need to generate single key provider source, from template.
739+
# Take provided key, in form of a string and make it into C array, BOOT_AES_RAW_KEY_HEX_ARRAY,
740+
# of byte size hex values.
741+
set(BOOT_AES_RAW_KEY_HEX_STRING ${BOOT_ENCRYPT_IMAGE_EMBEDDED_RAW_KEY})
742+
string(REGEX REPLACE "(..)" "0x\\1, " BOOT_AES_RAW_KEY_HEX_ARRAY "${BOOT_AES_RAW_KEY_HEX_STRING}")
743+
744+
# The tamplate references BOOT_AES_RAW_KEY_HEX_ARRAY where it expects the array to be substituted.
745+
set(OUTPUT_BOOT_AES_RAW_KEY_SRC ${ZEPHYR_BINARY_DIR}/mcuboot_generated/builtin_aes_key_provider.c)
746+
configure_file(templates/single_builtin_aes_key_provider.c.template ${OUTPUT_BOOT_AES_RAW_KEY_SRC} @ONLY)
747+
748+
# Add generated source file to build
749+
zephyr_library_sources(${OUTPUT_BOOT_AES_RAW_KEY_SRC})
750+
endif()

‎boot/zephyr/Kconfig‎

Lines changed: 60 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -94,6 +94,8 @@ config BOOT_ED25519_PSA_DEPENDENCIES
9494

9595
if BOOT_ENCRYPT_IMAGE
9696

97+
if !BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
98+
9799
config BOOT_X25519_PSA_DEPENDENCIES
98100
bool
99101
select PSA_WANT_ALG_ECDH
@@ -111,6 +113,8 @@ config BOOT_X25519_PSA_DEPENDENCIES
111113
to use with it; the others are used for shared key decryption
112114
and derivation.
113115

116+
endif # !BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
117+
114118
endif # BOOT_ENCRYPT_IMAGE
115119

116120
config BOOT_ECDSA_PSA_DEPENDENCIES
@@ -359,7 +363,7 @@ config BOOT_ED25519_PSA
359363
select BOOT_IMG_HASH_ALG_SHA256_ALLOW
360364
select BOOT_IMG_HASH_ALG_SHA512_ALLOW
361365
select BOOT_ED25519_PSA_DEPENDENCIES
362-
select BOOT_X25519_PSA_DEPENDENCIES if BOOT_ENCRYPT_IMAGE
366+
select BOOT_X25519_PSA_DEPENDENCIES if BOOT_ENCRYPT_IMAGE_WITH_SHARED_KEY
363367

364368
endchoice
365369

@@ -595,7 +599,8 @@ config BOOT_BOOTSTRAP
595599

596600
config BOOT_SWAP_SAVE_ENCTLV
597601
bool "Save encrypted key TLVs instead of plaintext keys in swap metadata"
598-
depends on BOOT_ENCRYPT_IMAGE
602+
depends on BOOT_ENCRYPT_IMAGE_WITH_SHARED_KEY
603+
depends on !BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
599604
help
600605
If y, instead of saving the encrypted image keys in plaintext in the
601606
swap resume metadata, save the encrypted image TLVs. This should be used
@@ -663,12 +668,62 @@ config BOOT_ENCRYPTION_SUPPORT
663668
help
664669
Hidden option used to check if image encryption is supported.
665670

666-
config BOOT_ENCRYPT_IMAGE
667-
bool "Support for encrypted image updates"
668-
depends on BOOT_ENCRYPTION_SUPPORT
671+
config BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
672+
bool "Use key that is already on board with MCUboot"
673+
depends on BOOT_ENCRYPT_IMAGE
674+
help
675+
The key is supposed to be either compiled in or on board.
676+
User is responsible for providing boot_enc_take_key
677+
function that will be able to retrieve the key.
678+
679+
if BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
680+
681+
choice BOOT_ENCRYPT_IMAGE_EMBEDDED_KEY_PROVIDER
682+
prompt "Embedded AES key provider"
683+
default BOOT_ENCRYPT_IMAGE_GENERATE_BASIC_KEY_PROVIDER
684+
685+
config BOOT_ENCRYPT_IMAGE_GENERATE_BASIC_KEY_PROVIDER
686+
bool "Generate basic boot_enc_take_key"
687+
depends on BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
688+
help
689+
Basic implementation of boot_enc_take_key will be implemented,
690+
that will have single key built in, used for all images and
691+
slots.
692+
693+
config BOOT_ENCRYPT_IMAGE_USE_CUSTOM_KEY_PROVIDER
694+
bool "User provides source code for key provider"
695+
help
696+
User is required to provide implementation for
697+
the boot_enc_take_key function.
698+
699+
endchoice # BOOT_ENCRYPT_IMAGE_EMBEDDED_KEY_PROVIDER
700+
701+
config BOOT_ENCRYPT_IMAGE_EMBEDDED_RAW_KEY
702+
string "Hexadecimal string representing AES key"
703+
depends on BOOT_ENCRYPT_IMAGE_GENERATE_BASIC_KEY_PROVIDER
704+
help
705+
AES key in form of hexadecimal string that will be used to
706+
generate boot_enc_take_key function, returning the key for
707+
decryption and encryption of image.
708+
The key character length should be the double of expected
709+
AES key length in bytes.
710+
711+
endif # BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
712+
713+
config BOOT_ENCRYPT_IMAGE_WITH_SHARED_KEY
714+
bool
715+
default y if !BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
716+
depends on BOOT_ENCRYPT_IMAGE
669717
select BOOT_ENCRYPT_RSA if BOOT_SIGNATURE_TYPE_RSA
670718
select BOOT_ENCRYPT_EC256 if BOOT_SIGNATURE_TYPE_ECDSA_P256
671719
select BOOT_ENCRYPT_X25519 if BOOT_SIGNATURE_TYPE_ED25519
720+
help
721+
Hidden option for default behaviour where AES encryption key
722+
is derived from Public Key Cryptography key exchange.
723+
724+
config BOOT_ENCRYPT_IMAGE
725+
bool "Support for encrypted image updates"
726+
depends on BOOT_ENCRYPTION_SUPPORT
672727
depends on !SINGLE_APPLICATION_SLOT || MCUBOOT_SERIAL
673728
help
674729
If y, images in the secondary slot can be encrypted and are decrypted

‎boot/zephyr/include/mcuboot_config/mcuboot_config.h‎

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -151,6 +151,11 @@
151151
#define MCUBOOT_USE_TLV_ALLOW_LIST 1
152152
#endif
153153

154+
#ifdef CONFIG_BOOT_ENCRYPT_IMAGE_WITH_EMBEDDED_KEY
155+
#define MCUBOOT_ENC_IMAGES
156+
#define MCUBOOT_EMBEDDED_ENC_KEY
157+
#endif
158+
154159
#ifdef CONFIG_BOOT_ENCRYPT_RSA
155160
#define MCUBOOT_ENC_IMAGES
156161
#define MCUBOOT_ENCRYPT_RSA

‎boot/zephyr/templates/single_builtin_aes_key_provider.c.template‎

Lines changed: 26 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,26 @@
1+
/*
2+
* SPDX-License-Identifier: Apache-2.0
3+
*
4+
* Copyright (c) 2025 Nordic Semiconductor ASA
5+
*
6+
*/
7+
8+
#include <stddef.h>
9+
#include <stdbool.h>
10+
#include <inttypes.h>
11+
#include <stdlib.h>
12+
#include <string.h>
13+
14+
#include "mcuboot_config/mcuboot_config.h"
15+
#include "bootutil/enc_key.h"
16+
17+
int boot_take_enc_key(uint8_t *key, int image, int slot)
18+
{
19+
const unsigned char array[] = {
20+
@BOOT_AES_RAW_KEY_HEX_ARRAY@
21+
};
22+
23+
memcpy(key, array, sizeof(array));
24+
25+
return 0;
26+
}

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